If you are consistently getting done times much earlier than expected...
- Are you using a temperature probe to measure your ambient box temp and comparing it to your knob setting? Your box temp might be running high.
- Test the accuracy of your probes by placing in boiling water. They should say 212.
- What part of the brisket are you temping? Flat? Point? Check the temp of various areas if you think the time doesn't seem right.
I have a #1, and only have to cut a very large brisket (14-18 lbs) into two pieces (point and flat). I think I might even be able to get a 10 lb brisket in there in one piece. Fewer pieces will mean more even cooking, and moister end product.
Either way, I have my doubts that you can get a brisket tender in 3 hours, no matter how small the piece. It just requires a certain amount of time to liquefy connective tissue into gelatin.